EP0225263A1 - Sheet or strip of stainless ferritic steel, particularly for exhaust systems - Google Patents

Abstract

The invention concerns a strip or sheet of ferritic stainless steel of the following composition (% by weight): (C+N)<.060; Si<0.9; Mn<1; Cr=15 to 19; Mo<1; Ni<0.5; Ti<0.1; Cu<0.4; S<0.02; P<0.045; Zr=0.10 to 0.40, and 7(C+N)</=Zr</=7(C+N)+0.15; Nb=0.25 to 0.55, in non-combined form; Al=0.020 to 0.080; Fe=the balance, in which Al is essentially in solid solution. The process for the production of the sheet or strip comprises a final annealing operation which is carried out at between 980 DEG and 1020 DEG C., typically for 0.5 to 5 minutes at between 990 DEG and 1010 DEG C. The strip or sheet according to the invention is used for any application requiring an economic compromise in regard to ductility (sheet and welds), hot resistance and resistance to corrosion, for example in motor vehicle exhaust manifolds.

Description

The invention relates to the field of rolled products of stainless ferritic steel, and more particularly that of exhaust systems.

EXPOSURE OF THE PROBLEM

• - bonne ductilité des soudures sans métal d'apport pour la conformation en pièces et la fatigue,
• - bonne résistance au fluage à chaud (SAGTEST);
• - bonne résistance à l'oxydation à chaud, continue ou cyclique, aussi bien pour la pleine tôle que pour les soudures;
• - bonne résistance à la corrosion, en particulier vis-à-vis des projections d'eau salée provenant du salage des routes pendant l'hiver.

The manufacture and maintenance in service of exhaust systems, for example automobile exhaust manifolds and tubes, has a number of requirements which are difficult to meet simultaneously and economically:

• - good ductility of welds without filler metal for component shaping and fatigue,
• - good resistance to hot creep (SAGTEST);
• - good resistance to hot oxidation, continuous or cyclic, both for full sheet metal and for welds;
• - good resistance to corrosion, in particular vis-à-vis salt water projections from road salting during winter.

The Applicant has tried to improve the properties compromises obtained with sheets or strips of shades already known and more particularly with stainless ferritic grades stabilized with Nb and / or Zr, and this especially with regard to the creep resistance and to hot oxidation. Ferritic stainless grades are superior to austenitic grades because of their coefficient of thermal expansion.

KNOWN STATE OF THE ART

• - l'ordre de facilité de formation des nitrures et carbures est le suivant: "nitrure de Zr, carbure de Zr, carbure de Nb et nitrure de Nb", C et N sont donc piégés préférentiellement par Zr, et de façon plus stable que par Nb;
• - le zirconium dépassant de plus de 0,25 % la quantité requise pour se combiner avec C et N entraîne une détérioration sensible de la ductilité et de la résistance à la corrosion;
• - le niobium en solution solide ne doit pas excéder 0,3 X sinon il conduit à une mauvaise ductilité des soudures, la fragilité pouvant être due à la formation d'un composé intermétallique Nb₂ (Fe,Cr)₃.

US-A-4: 010 049 relates to a ferritic stainless steel of composition: C 0.10% maximum; Cr 11 to 30%; Mo 3% max; Nb (columbium) 0.1% total at 0.3 X in solid solution and not less than 7.7xC% - (Zr% -6.5xN Z); Zr 6.5 N% at 0.25 X + (7.6% C + 6.5% N); Fe and residual impurities the balance. This document gives several indications on the role of Zr and Nb:

• the order of ease of formation of the nitrides and carbides is as follows: "Zr nitride, Zr carbide, Nb carbide and Nb nitride", C and N are therefore preferably trapped by Zr, and in a more stable manner than by Nb;
• - the zirconium exceeding by more than 0.25% the quantity required to combine with C and N results in a significant deterioration of the ductility and of the corrosion resistance;
• - the niobium in solid solution must not exceed 0.3 X otherwise it leads to poor ductility of the welds, the brittleness possibly being due to the formation of an intermetallic compound Nb ₂ (Fe, Cr) ₃ .

In the communication presented to the Congress of the S.A.E. in Detroit, Michigan on February 23-27, 1981 ("Influence of Columbium on the 870 ° C Creep Properties of 18 Z Chromium Ferritic Stainless Steels"), John N. JOHNSON studies the creep resistance at traction at 870 ° C various ferritic stainless steels at 18 X Cr containing T + Nb and notes that the creep results are improved by re-annealing the samples (already annealed in the factory) at temperatures ranging from 1040 to 1150 ° C, for example 30 min at 1095 ° C. The materials having contents of uncombined Nb of 0.3 to 0.6% comprise many intergranular precipitates based on Nb and the effect of the second annealing was to dissolve these precipitates and to increase the grain size.

Two documents indicate the influence of an addition of A1. Patent application FR-A-2 463 194 relates to ferritic steels with 1 to 20% Cr and Ti, Nb with 0.5 to 2% Al, in which a minimum Al content of 0.5% and preferably 0.75% is required to provide resistance to oxidation at elevated temperatures. Above 2%, Al has a detrimental effect on the weldability. In patent application JP-A-82/146440, a ferritic stainless steel contains 0.08 to 0.5% A1 and one or more of the elements B (2-50 ppm), Ti (0.005-0.4 X) , Nb (0.005-0.4 X), V (0.005-0.4%) and Zr (0.005-0.4%), and Al causes structural changes at the various stages of sheet metal transformation with an increase "ridging resistance" (resistance to the phenomenon of stringing or crumpling).

STATEMENT OF THE INVENTION

• (C+N) < 0,060 - Si < 0,9 - Mn < 1
• Cr 15 à 19 - Mo < 1 - Ni < 0,5 - Ti < 0,1 - Cu < 0,4 - S < 0,02 - P < 0,045
• Zr = 0,10 à 0,50 avec Zr compris entre 7 (C+N) - 0,1 et 7 (C+N) + 0,2
• Nb compris entre 0,25 et 0,55 si Zr ≥ 7 (C+N)
• et compris entre 0,25 + 7 (C+N) - Zr et 0,55 + 7 (C+N) - Zr si Zr < 7 (C+N)
• Al 0,020 à 0,080 - autres éléments et Fe : le solde.

The subject of the invention is a sheet or strip of ferritic stainless steel, usually in the annealed state, the final annealing operation then being most often followed by a finishing and work hardening pass or "skin- pass "producing an elongation of less than 1 X, and intended in particular for the manufacture of manifolds and exhaust tubes. This sheet or strip has the composition (% by weight):

• (C + N) <0.060 - If <0.9 - Mn <1
• Cr 15 to 19 - Mo <1 - Ni <0.5 - Ti <0.1 - Cu <0.4 - S <0.02 - P <0.045
• Zr = 0.10 to 0.50 with Zr between 7 (C + N) - 0.1 and 7 (C + N) + 0.2
• Nb between 0.25 and 0.55 if Zr ≥ 7 (C + N)
• and between 0.25 + 7 (C + N) - Zr and 0.55 + 7 (C + N) - Zr if Zr <7 (C + N)
• Al 0.020 to 0.080 - other elements and Fe: the balance.

Zr is consumed by stabilization, that is to say by trapping C and N in the form of nitrides and carbides, up to approximately 7 (C + N)%. The free Zr is therefore limited to 0.2%, which makes it possible to avoid the drawbacks linked to the formation of eutectic compounds containing Fe ₃ Zr in the case where there is more than 0.25% of free Zr, these compounds resulting in a degradation of the mechanical characteristics, in particular of the ductility and of the creep resistance, and a reduction in the corrosion resistance, as indicated for the most part by document US-A-4,010,049.

At this content at most equal to 0.2%, the free Zr has no significant direct influence on the resistance to oxidation.

The free or non-combined Nb is between 0.25 and 0.55%. The total Nb includes in addition to the free Nb an addition of 7 (C + N) -Zr, to compensate for the lack of stabilization by insufficient Zr, in the case where Zr is between 7 (C + N) - 0.1 and 7 (C + N).

It is known by JOHNSON (document cited above) that the free or uncombined Nb increases the creep resistance at the level of 0.3 to 0.6% when the tested samples have been annealed at at least 1040 ° C. However, it has been observed, in annealing tests on sheets according to the invention with a thickness of 1 mm vertical, at 1040 ° C. for 5 min and at 1150 ° C. for 1 min, that it occurs with annealing temperatures. as high an unacceptable creep deformation of the sheet. And we observed that an annealing of 1000 ° C 1 min resulted in good creep and ductility results for the sheets of the invention, and that in general annealing at 1000 + 10 ° C of duration between 0.5 and 5 min was suitable, which is industrial implementation much easier than annealing at a temperature of at least 1040 ° C. As for the poor ductility of the welds reported by US-A-4,010,049, when Nb in solid solution (that is to say free or not combined) is in content greater than 0.3%, and this in the case of 18% Cr stainless steel ferritic sheets with Nb + Zr, this drawback does not exist with the sheets of the invention whose TIG welds without filler metal are extremely ductile.

The total content of A1 assayed essentially corresponds to Al in solid solution. Indeed, Zr has more affinity than Al for oxygen and there is little residual oxygen in the metal, so that there can be only very little Al in the form of alumina. Furthermore, the affinities of Zr and Nb for nitrogen and the greater affinity of Al for oxygen than for nitrogen means that aluminum nitride AIN does not form. The result; confirmed qualitatively by micrographic examinations, is that A1 is in solid solution with the exception of a content at most equal to 0.003% and corresponding essentially to alumina. With a small addition of 0.020 to 0.080% of A1, a surprising improvement in the resistance to hot oxidation is obtained, linked to the role of aluminum in solid solution, whether it is continuous oxidation. in air between 800 and 1000 ° C or alternating cyclic oxidation in full sheet metal or on welds. Thus, in continuous oxidation in air for 50 hours, the limit temperatures corresponding to a weight loss of 200 g / m ² are 970 ° C for Al <0.002%, 1020 ° C for Al = 0.036% and 1070 ° C for Al = 0.090%. And in continuous oxidation at 1000 ° C for 50 h, the following weight losses are obtained:

Weight loss is thus reduced by 26% by the presence of 0.020 Z of Al and by 53% by the presence of 0.080% of Al. The Al content is limited to 0.080% so as to avoid, as has been the experience with stainless steels at 17% Cr (type AISI 430), dross on the weld beads causing irregular oxidation and cracking at the conformation therefore faster corrosion. This dross effect is significant at the level of 0.1% Al; but if one also wants to avoid or limit the inclusions of alumina linked to the presence of too much aluminum, these inclusions being sites of initiation of stings following the projections of salt water coming from salting or desalination of roads during the winter, it is advisable to remain below 0.05% of Al as it is done in the preferred composition indicated below.

We sought to explain this surprising improvement in the resistance to oxidation produced by such low aluminum contents. The study was carried out on sheet samples of thickness 1 mm coming in particular from two flows at 16% Cr without Zr, n ° 101 and n ° 401, one at 0.6 Z Nb and 0.048 Z Al, and the other at 0.45 Z Nb without Al (Al <0.002 X), continuously oxidized in air for 50 h at 900 ° C. In the case of the first casting containing Nb and Al, it was observed that the oxidized layer of thickness 10 μm was anchored to the sheet by small plates of typical unit dimensions 0.3 to 0.8 μm, containing alumina and in places of niobium, in the form of inclusions of Nb compound. This anchoring mechanism is quite different from the mechanism for forming an alumina layer which is specific to stainless ferritic grades with an Al content greater than 0.5 X.

In the case of the second casting without Al, there is no anchoring and, in glow discharge spectrographic examination, it has been verified that there was no Nb at the metal / oxidized layer interface.

It can therefore be concluded that, in the case of the sheets of the invention with Zr-Nb-Al, Al appears to intervene in conjunction with Nb to produce an anchoring favorable to the resistance of the oxidized layer and thus improve the resistance to corrosion. hot. Furthermore, in a series of alternating oxidation tests at 800 ° C., sheet metal samples according to the invention with Zr-Nb-Al showed beyond 350 h of test better resistance than samples of t81e to Nb-A1 without Zr having comparable non-combined Nb and Al contents, which seems to show that Zr has a role in this resistance to alternating or cyclic oxidation.

• (C+N) < 0,040 - Si < 0,8 - Cr 16 à 18
• Mo < 0,3 - Ni < 0,3 - Ti < 0,05 - S < 0,01
• Zr = 0,10 à 0,40 avec Zr compris entre 7 (C+N) et 7 (C+N) + 0,15
• Nb 0,30 à 0,52 et encore de préférence 0,33 à 0,50
• Al 0,020 à 0,045 et encore de préférence 0,025 à 0,040.

The constituent elements of the tale or strip of the invention are taken individually or as a whole in the following preferential content ranges:

• (C + N) <0.040 - If <0.8 - Cr 16 to 18
• Mo <0.3 - Ni <0.3 - Ti <0.05 - S <0.01
• Zr = 0.10 to 0.40 with Zr between 7 (C + N) and 7 (C + N) + 0.15
• Nb 0.30 to 0.52 and more preferably 0.33 to 0.50
• Al 0.020 to 0.045 and more preferably 0.025 to 0.040.

The maximum contents of (C + N) and Zr can thus be lowered simultaneously, giving greater security for the ductility of the full sheet and welds as well as for corrosion resistance. Zr is then always in sufficient quantity for stabilization in the restricted sense, that is to say for the trapping of N and C in the form of nitrides and carbides. Nb is fully available for resistance to hot creep and is included in the content ranges which give the minimum deflection for SAGTEST tests at 850 ° C. Al can be included in increasingly narrow ranges, industrially achievable and representing an optimal compromise between resistance to hot oxidation and resistance to pitting corrosion.

In the delivery state, the sheet or strip according to the invention is in the annealed and optionally erected state, this annealed state typically corresponding to a treatment at 1000 ± 10 ° C for 0.5 to 5 min.

The invention also relates to the method of manufacturing a sheet or strip of ferritic stainless steel, in which, as is known, the hot rolled strip is annealed with a thickness of between 2.5 and 5 mm between 800 and 1000 ° C. under slightly oxidizing conditions, then it is shot and pickled, then it is cold rolled to the delivery thickness typically between 0.6 and 3 mm, with or without annealing and intermediate pickling, and it is annealed at the end of the parade, then it is subjected to a finishing pass or so-called "skin-pass" pass producing an elongation of less than 1 i, with possibly a final pickling. This process differs from the prior art in that the sheet or strip has the composition of the invention and in that the annealing, making it possible to obtain good hot creep results, is carried out between 980 and 1020 ° C. , and preferably between 990 and 1010 ° C for 0.5 to 5 min, or at a temperature and for a time giving an equivalent metallurgical state. This re final baking is typically carried out following a rolling producing an elongation of at least 100% from the previous annealing.

The test results which will follow, the figures and the tables which accompany them, will make it possible to illustrate and comment on the various aspects of the invention.

TESTS . Test series n ° 1 - Hot creep tests

A number of laboratory castings of 25 kg each were carried out, the analyzes of which are shown in Table 2 (Nb + Al castings) and in Table 3 (Zr + _N b + Al castings).

Other impurities were analyzed: W <0.003 - V = 0.02 to 0.06 - Sn <0.003 - Co 0.01 to 0.02 - Ti = 0.004 to 0.013 - Pb <0.002 - Ta <0.01 Se <0.002 - Mg <0.0002 - Ca = 0.0001 to 0.0003 - 0 = 0.0036 to 0.0172%.

The total of the other impurities is thus clearly less than 0.3% and Fe constitutes the balance.

• - forgeage à chaud à épaisseur 14 mm,
• - rectification des 2 faces à épaisseur 12 mm,
• - laminage à chaud en épaisseur 3 mm,
• - recuit 4 h à 800°C,
• décapage,
• - laminage à froid à épaisseur 1 mm,
• - recuit à 1000°C soit 1 mn, soit 5 mn.

The main stages of the transformation into 1 mm thick sheets were as follows:

• - 14 mm thick hot forging,
• - rectification of the 2 faces with a thickness of 12 mm,
• - hot rolling in thickness 3 mm,
• - annealing 4 h at 800 ° C,
• pickling,
• - cold rolling 1 mm thick,
• - annealing at 1000 ° C, either 1 min or 5 min.

Rectangular test specimens of 310 x 25 mm were cut from the sheets, and bent at 90 ° to 25 mm from one end. Then they were placed flat each on 2 supports of internal distance 254 mm and external distance 264 mm and they were subjected to continuous SAGTEST tests of creep under their own weight during 100 h at 850 ° C.

The graphs in Figures 1 and 2 show the arrows observed after 100 h at 850 ° C, and Tables 4 and 5 show the average arrows (means of 3 results) obtained for the test pieces in Table 2 and for those in Table 3 .

• - les éprouvettes au Nb+A1 (Tableau 4 et fig. 1 et 2) résistent d'autant mieux au fluage que la teneur en Nb libre est élevée, et le recuit préalable de 5 mn à 1000°C donne, à même teneur en Nb libre, des résultats bien meilleurs que le recuit de 1 mn à 1000°C, cela pour tout l'intervalle testé (0,1 à 0,54 % de Nb libre);
• - les éprouvettes au Zr+Nb+Al (tableau 5 et fig. 1 et 2) donnent des résultats bien meilleurs que les éprouvettes au Nb+Al avec le recuit de 1mn à 1000°C, et qui ne sont qu'un peu moins bons- que ceux obtenus avec les éprouvettes semblables au Zr+Nb+Al recuites 5 mn à 1000°C, spécialement entre 0,25 % et 0,55 % de Nb libre, intervalle dans lequel les résultats de flèches ne diffèrent que de environ 0,3 à 0,7 cm. L'aptitude des coulées au Zr+Nb+Al selon l'invention à donner une relativement bonne résistance au fluage même après un recuit limité de la sorte (1 mn à 1000°C) est un avantage industriel très important;
• - les éprouvettes au Zr+Nb+A1, qu'elles aient été recuites 1 mn ou 5 mn à 1000°C, ont une résistance maximale au fluage à chaud (SAGTEST à 850°C-100 h), c'est-à-dire une flèche minimale entre 0,30 et 0,52 % de Nb libre ou non combiné, ou mieux entre 0,33 et 0,50 % de Nb libre.

These results show three trends:

• - the Nb + A1 test pieces (Table 4 and Fig. 1 and 2) resist creep the better the free Nb content is high, and the prior annealing of 5 min at 1000 ° C gives the same content in Nb free, much better results than annealing for 1 min at 1000 ° C, for the entire range tested (0.1 to 0.54% of free Nb);
• - Zr + Nb + Al test tubes (table 5 and fig. 1 and 2) give much better results than Nb + Al test tubes with 1 min annealing at 1000 ° C, and which are only slightly less good - than those obtained with test pieces similar to Zr + Nb + Al annealed for 5 min at 1000 ° C, especially between 0.25% and 0.55% of free Nb, the range in which the arrow results differ only by approximately 0.3 to 0.7 cm. The ability of Zr + Nb + Al castings according to the invention to give a relatively good creep resistance even after such limited annealing (1 min at 1000 ° C) is a very important industrial advantage;
• - Zr + Nb + A1 test pieces, whether they have been annealed for 1 min or 5 min at 1000 ° C, have maximum resistance to hot creep (SAGTEST at 850 ° C-100 h), i.e. - say a minimum deflection between 0.30 and 0.52% of free or uncombined Nb, or better still between 0.33 and 0.50% of free Nb.

The appearance of the curves concerning Zr + Nb + Al flows, different from those concerning Nb + Al flows (fig. 1 and 2), is not completely explained by the classic considerations and observations of intergranular phase precipitation iron-niobium intermetallic and recrystallization.

The sheets according to the invention, with an uncombined Nb content of between 0.25 and 0.55%, and preferably with the intervals specified above, are thus indicated by their level of resistance to hot creep. Tensile creep tests at 800 ° C have confirmed the tests in this regard.

. Test series n ° 2 - Ductility of welds

2.5 mm thick sheets were used which were annealed for 4 h at 800 ° C. from 4 of the previous castings. On these sheets, full sheet welds were made (melting lines) with a back width of 2 mm in automatic IIG on a strip with a groove width of 10 mm, under pure argon, with 12V-250 A and at a speed of 0.50 m / min. . We then carried out successive bending tests of the welds, either in the transverse direction of the welds, or in the long direction: at 90 ° angle then at 180 ° on a radius of 5 mm, then on a radius of 2.5 mm, then at block (zero radius).

The results are shown in Table 6 below, "B" meaning "good" and "M" meaning "bad" (crack or crack). There are 4 tests per condition.

The welds of the sheets according to the invention all show very good ductility, unlike sheets with Nb + Al, and this even in the case of casting No. 445 where the Zr content is slightly less than 7 (C + N).

. Test series n ° 3 - Continuous air oxidation tests between 800 and 1050 ° C

The samples used come from 3 castings of 25 kg transformed according to the process indicated in connection with test series No. 1, the cold rolling being stopped at the thickness of 1.5 mm and being followed by annealing under vacuum. 1 hour at 830 ° C. The analyzes of the 3 castings, at 0.4 Z Nb and with increasing Al content, are shown in Table 7. The samples are 20 × 30 mm plates cut out with a punch in the annealed 1.5 mm sheets, then electrolytically polished in an aceto-perchloric bath (88-12) at room temperature for 5 min, then weighed in mg. Each oxidation test relates to 3 test pieces of the same type, with an additional test piece for metallographic examinations.

The hot air oxidation tests have a unit duration of 50 h, the air renewal is ensured by "chimney effect" by means of a 0 6 mm hole made in the lower part of the oven. After testing, the oxides formed are removed by electrolytic pickling in a neutral medium, and it is the weight loss of the samples per unit area (in g / m ² ) which makes it possible to assess "a contrario" the resistance to hot oxidation. The results on the 3 test pieces of each test are well grouped in the case of continuous oxidation, and we have therefore given in this case only one result, average of the 3 individual results.

En considérant le Tableau 8 et la figure 3, on voit que Al en teneur aussi faible que 0,036 % améliore beaucoup la résistance à l'oxydation à chaud au-dessus de 950°C. Et, si l'on prend par exemple comme limite 200 g/m² en 50 h, on voit que les températures limites s'étagent comme il a déjà été indiqué dans l'exposé de l'invention. Comme cela va se vérifier dans la série d'essai n° 4, ces observations sur des coulées au Nb s'appliquent aux coulées au Zr+Nb et en particulier aux coulées selon l'invention.This series of tests No. 3 focused on oxidations at temperatures ranging from 50 ° C to 50 ° C between 800 ° C and 1050 ° C, the results appear in Table 8 and in Figure 3.

Considering Table 8 and Figure 3, it can be seen that Al in a content as low as 0.036% greatly improves the resistance to hot oxidation above 950 ° C. And, if we take for example as a limit 200 g / m ² in 50 h, we see that the temperature limits are stepped as already indicated in the description of the invention. As will be verified in test series No. 4, these observations on Nb flows apply to Zr + Nb flows and in particular to flows according to the invention.

. Test series n ° 4 - Continuous air oxidation tests for 50h at 1000 ° C

In this series, in addition to test pieces from the 3 previous flows, we tested test pieces of 2 flows with Nb of respective A1 contents of 0.525% and 1% and of two flows according to the invention at 0.04% Al (flows 201 and 202), the analyzes of which are also given in Table 7. The results are given in Table 9 and in FIG. 4. It can be seen that the representative points of the two flows according to the invention are correctly placed on the weight loss curve plotted for the Nb flows. The presence of aluminum leads to a weight loss reduced by 50% by 0.04% A1, by 80X by 0.10% Al and which hardly changes beyond 0.3% Al, the loss weight then capped at 100 g / m ² , asymptote of the curve. The weight losses corresponding to the Al content limits of the steels of the invention are shown in Table I (description of the invention).

The Al contents are plotted on the graph in FIG. 4.

. Test series n ° 5 - Alternate oxidation tests between 800 and 100 ° C, in full sheet metal and on weld

In these alternating oxidation or cyclic oxidation tests, the test pieces prepared as described above are subjected to cycles each comprising: rapid heating, holding for 10 min at the test temperature, then air cooling and maintenance at room temperature or close to total duration 10 min. The duration of a test is 100 h during which 300 cycles are carried out giving an overall maintenance at the test temperature of 50 h.

We tested in this way, with test temperatures staggered from 50 ° C to 50 ° C between 800 ° C and 1000 ° C, test specimens from Nb + Al n ° 101 casting and castings according to the invention n ° 201 and 202, the analyzes of which appear in Table 7. The tests relate both to full-sheet test pieces and to test pieces containing welds, the latter being carried out as indicated with regard to the series of tests No. 2, and the right side of these welds then occupying 1/3 of the width of the test pieces.

• - les points représentatifs des éprouvettes pleine tôle des 3 coulées et ceux des éprouvettes avec soudure des coulées n° 201 et 202 selon l'invention, appartenant au domaine hachuré (A);
• - les points représentatifs des éprouvettes avec soudure de la coulée n° 101, qui présente une anomalie de perte de poids (oxydation excessive dans ce test cyclique) entre 850 et 950°C et donne des pertes de poids relativement fortes pour 1000°C. Ces points sont compris dans le domaine hachuré (B).

The results obtained appear in Table 10 and in Figure 5. The points representative of the minima and maxima of each group of 3 results are plotted in Figure 5. There are two families of results:

• - the representative points of the full sheet test pieces of the 3 castings and those of the test pieces with welding of the n ° 201 and 202 castings according to the invention, belonging to the hatched area (A);
• - the representative points of the test pieces with welding of casting no. 101, which has an anomaly in weight loss (excessive oxidation in this cyclic test) between 850 and 950 ° C and gives relatively strong weight losses for 1000 ° C. These points are included in the hatched area (B).

With the same Al content, the sheets of the invention are therefore distinguished from sheets of Nb without Zr in that their welds without filler metal have better resistance to alternating or cyclic oxidation in this temperature range (850 to 950 ° C) important for exhaust manifolds.

. Test series n ° 6 - Alternate oxidation tests of 500 h at 800 ° C

In this series of tests, alternating or cyclic oxidation tests were carried out for a total duration of 100 h, 250 h and 500 h with the cycles defined in test series No. 5. The tests focused on the flows No. 101 102 and 201 (analyzes in Table 7): Nb, Zr, and Zr + Nb flows respectively according to the invention, having neighboring Al contents.

The results obtained, already mentioned in the description of the invention, are reported in Table 11 and in FIG. 6. The results of flows # 101 and 201 for 100 h (at 800 ° C.) are already shown in the Table 10. It is observed that the evolution of the weight loss as a function of the alternating or cyclic oxidation time is quite different for the 3 flows: flow 201, which gave the greatest weight losses at 100 h, gives weight losses well grouped and practically no longer evolving beyond 100 h to 250 h, while the flows n ° 101 and especially n ° 102 give results in strong increase with the data. Flow no. 201 according to the invention here outperforms flow no. 101 and 102 after approximately 350 hours of testing.

This behavior of the test pieces of casting No. 201, corresponding to a particular stability of the oxide layer in cyclic oxidation, seems to confirm that this stability which seems linked to an anchoring phenomenon, does not only depend on the presence of the 'aluminum. Compared with the behavior of the specimens from flows 101 and 102, it seems to mean that the simultaneous presence of Zr and Nb also play a role.

ADVANTAGES OF THE INVENTION

• a) Bonne résistance au fluage à chaud, particulièrement pour 0,30 à 0,52 % de Nb. libre;
• h) cette résistance au fluage est obtenue à partir d'un état recuit industriellement avantageux, typiquement 1000 _t 10°C pendant 0,5 à 5 mn;
• c) bonne résistance à l'oxydation continue à chaud, liée de façon surprenante à l'addition d'Al en faible teneur, en conjonction avec Nb;
• d) stabilité particulière de la couche d'oxyde en oxydation cyclique à 800°C, liée à la présence simultanée de Zr et de Nb en même temps que Al en faible teneur;
• e) bon comportement des soudures sans métal d'apport en oxydation cyclique particulièrement aux alentours de 900°C, ce comportement restant voisin de celui de la pleine tôle;
• f) bonne ductilité des soudures sans métal d'apport;
• g) bonne résistance à la corrosion, dans des conditions correspondant à l'utilisation des collecteurs d'échappement automobile, grâce à la limitation de la teneur en Al.

The sheets of the invention thus have numerous advantages responding to the problem posed:

• a) Good resistance to hot creep, particularly for 0.30 to 0.52% of Nb. free;
• h) this creep resistance is obtained from an industrially advantageous annealed condition, typically 1000 _{t at} 10 ° C. for 0.5 to 5 min;
• c) good resistance to continuous hot oxidation, surprisingly linked to the addition of Al in low content, in conjunction with Nb;
• d) particular stability of the oxide layer in cyclic oxidation at 800 ° C, linked to the simultaneous presence of Zr and Nb at the same time as Al in low content;
• e) good behavior of welds without filler metal in cyclic oxidation, particularly around 900 ° C., this behavior remaining close to that of full sheet metal;
• f) good ductility of welds without filler metal;
• g) good resistance to corrosion, under conditions corresponding to the use of automobile exhaust manifolds, thanks to the limitation of the Al content.

APPLICATIONS

The strips or sheets of the invention, usually in the annealed state and in a thickness of 0.6 to 3 mm and most often 1.2 to 2.5 mm, are used for any application in which an economic compromise of ductility (sheet metal and welds), heat resistance (creep, continuous or cyclic air oxidation) and resistance to corrosion. Application to exhaust systems is particularly typical.

Claims (5)

1. Sheet or strip of ferritic stainless steel, intended in particular for the manufacture of exhaust systems, characterized in that it has the composition (X by weight) (C + N) <0.060 - Si <0.9 - Mn <1-Cr 15 to 19 - No <1 - Ni <0.5, Ti <0.1 - Cu <0.4 - S <0.02 - P <0.045 - Zr = 0.10 to 0.50 with Zr between 7 (C + N) - 0.1 and 7 (C + N) + 0.2 - Nb between 0, 25 and 0.55 if Zr ≥ 7 (C + N), and between 0.25 + 7 (C + N) - Zr and 0.55 + 7 (C + N) - Zr if Zr <7 (C + NOT) - Al 0.020 to 0.080 - other elements and Fe: the balance, and in that Al is in solid solution with the exception of a content at most equal to 0.003 X. 2. Sheet or strip according to claim 1, characterized in that it contains (% by weight) C + N <0.040 - Si <0.8 - Cr 16 to 18 - Mo <0.3 - Ni <0.3 - Ti <0.05 - S <0.01 - Zr = 0, ₁₀ to 0.40 with Zr between 7 (C + N) and 7 (C + N) + 0.15-Nb 0.30 to 0.52 - Al 0.020 to 0.045. 3. Sheet or strip according to claim 2, characterized in that it contains (X th weight): Nb 0.33 to 0.50 and Al 0.025 to 0.040. 4. Sheet or strip according to any one of claims 1 to 3, characterized in that it is in the annealed state 0.5 to 5 min at 1000 ± 10 ° C and erected. 5. A method of manufacturing a sheet or strip, the composition of which conforms to any one of claims 1 to 3, in which the hot-rolled strip of thickness between 2.5 and 5 mm is annealed between 800 and 1000 ° C under slightly oxidizing conditions then it is shot and pickled, it is then cold rolled to the delivery thickness of between 0.6 and 3 mm, with or without annealing and intermediate pickling, and the final annealing on parade and it is subjected to a finishing pass and work hardening or "skin-pass" producing an elongation of less than 1%, characterized in that the final annealing is carried out between 980 and 1020 ° C. . 6. Method according to claim 5, characterized in that the final annealing is carried out between 990 and 1010 ° C for 0.5 to 5 min.

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